Spawning Cube on Same Level
When adding a new mesh/object into Blender, it will be added where your 3D cursor is (looks like a red and white circle). You can move the location of the 3D cursor by left clicking. Also, if you want to move the objects so that they are the same level, change their position on the Z-axis. This can be done in multiple ways


The properties window: by default, the properties window will be on the right side, under Outliner tab. To move the 'level' of an object (where it is located on the Z-axis), click on the Object tab (looks like a cube) and change the Z value under location (1st is X-value, 2nd is y-value, 3rd is Z-value). To put all your objects on the same level, make sure all of their Z-values are the same. 
The other properties tab: By pressing the key shortcut "N" or dragging the + (top right of your 3D view box) to the left, you can bring up the other properties tab. The Z-value of the objects location will be under 'Transform'.